"Gas" engines in the context of the present description are those designed for operation with a fuel that is gaseous under normal ambient conditions. The fuel can be any of various hydrocarbon gases, carbon monoxide, hydrogen, and mixtures of two or more gases. Such engines are frequently used in stationary applications and use fuel supplies where the composition of the fuel can vary considerably, including landfills and pipelines. Typical engine ratings can be, for example, 50 HP to 5000 HP. The air to fuel ratio of an engine running on a gas fuel is adjusted to load and speed. Where the fuel has a varying composition, its composition and molecular weight are usually not known. Variation in the fuel composition may result in undesirably high NOx emissions.
U.S. Pat. No. 4,867,127, Sep. 19, 1989, proposes a system based on the idea of adjusting the air to fuel ratio .lambda. (lamda) based on reading the air and fuel mixture pressure upstream of the inlet valves to the engine. For example, if a constant engine output is required, an increase in that pressure can be caused by an increase in .lambda. (or a leaning of the mixture). In such a system, all the relevant input signals for the control of .lambda. are sensed on the input side of the engine.